Group inventory: add atomic trainer-to-group transfer route

[prodmodfour]

Goal

Move item quantities from a trainer sheet back into the shared group inventory atomically.

Scope

• Add a use case and API route for trainer-to-group transfers.
• Accept trainerSlug, trainerRevision, groupSlug, groupRevision, section, a trainer row identifier or row index, and quantity.
• In one SQLite transaction:
  • read the trainer sheet and group inventory;
  • validate both revisions;
  • decrement/remove the trainer row;
  • merge the item into the group inventory section;
  • increment both document revisions;
  • return the authoritative trainer sheet update and group inventory.
• Start with GM-only access in this ticket.

Acceptance criteria

• Successful transfer updates both documents atomically.
• If either revision is stale, neither document changes.
• Invalid quantity, missing row, missing group inventory, and equipment partial-transfer cases are rejected clearly.
• Tests cover stackable and equipment transfers.

Depends on

• Group inventory: add SQLite repository with revision-checked updates #29
• Group inventory: add shared item merge and quantity transfer helpers #37
• Group inventory: add atomic group-to-trainer transfer route #38